1. Field of the Invention
This invention relates to registration of sheets in a feed path. It particularly relates to systems for controlling driving rolls to effect registration and deskewing of copy sheets.
2. Description of Related Art
Conventional sheet registration mechanisms for equipment using paper feed stocks, such as electrophotographic reproduction equipment and printers, use crossed nip rolls in conjunction with fixed guides or gates for positioning the copy sheet. The crossed nip rolls push the sheets against such guides and gates. These conventional systems have many drawbacks. If the sheet is driven against the guide and gate surfaces with excessive force, the edges of the sheets can be bent or crumpled. This condition occurs especially with light weight papers and causes problems in downstream feeding of the paper. Thus, each system must be carefully set for a narrow range of paper weight to provide sufficient driving force for movement of the sheet without damaging it as it is driven against a guide or gate. Further, the guides are subject to wear resulting from the impingement of the edges of a large number of copy sheets. Thus, the guides must be replaced one or more times during the life of a machine, thereby increasing maintenance costs. In addition, undesirable dust is formed as a result of the impact and sliding the paper against the hard guide surfaces. One system of this type, using a passive differential between spaced rolls to allow one of the rolls to rotate at a higher speed than the other of the rolls, has been proposed.
Sheet registration systems without mechanical guides or gates, using drive rolls for sheet registration, have also been proposed. Sheet registration stations of this type are shown in U.S. Pat. Nos. 4,438,917, 4,511,242, 4,971,304 and 5,049,442. In these sheet registration stations, the copy sheet is positioned longitudinally and laterally (registered) and the angular position of the copy sheet (skew) is corrected by driving two coaxial nip rolls at different speeds or by driving one roll at a constant speed and varying the drive speed of the other roll, so that one side of the sheet travels farther than the other, thus changing the angular position of the paper. Each of the two rolls is driven by an independently controlled motor. These designs present control limitations because of the requirements of precisely controlling the velocity difference between the two rolls (to achieve high positional resolution) and operating the drives at high speed (to achieve high throughput rates). This results from the fact that the drive system must accomplish two different functions with conflicting requirements. The rolls must transport the copy paper at relatively high speed, a condition in which the speed of both rolls must be exactly the same. The rolls must also correct the angular position of the copy sheet by precisely changing the relative angular position between the two rolls.